linux-kernel - Re: [PATCH] KVM: x86: work around leak of uninitialized stack contents

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Date:   Thu, 12 Sep 2019 14:20:09 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Fuqian Huang <huangfq.daxian@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: work around leak of uninitialized stack contents

On Wed, Sep 11, 2019 at 9:18 PM Fuqian Huang <huangfq.daxian@...il.com> wrote:
>
> Emulation of VMPTRST can incorrectly inject a page fault
> when passed an operand that points to an MMIO address.
> The page fault will use uninitialized kernel stack memory
> as the CR2 and error code.
>
> The right behavior would be to abort the VM with a KVM_EXIT_INTERNAL_ERROR
> exit to userspace; however, it is not an easy fix, so for now just ensure
> that the error code and CR2 are zero.
>
> Signed-off-by: Fuqian Huang <huangfq.daxian@...il.com>
> ---
>  arch/x86/kvm/x86.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 290c3c3efb87..7f442d710858 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5312,6 +5312,7 @@ int kvm_write_guest_virt_system(struct kvm_vcpu *vcpu, gva_t addr, void *val,
>         /* kvm_write_guest_virt_system can pull in tons of pages. */
>         vcpu->arch.l1tf_flush_l1d = true;
>
> +       memset(exception, 0, sizeof(*exception));
>         return kvm_write_guest_virt_helper(addr, val, bytes, vcpu,
>                                            PFERR_WRITE_MASK, exception);
>  }
> --
> 2.11.0
>
Perhaps you could also add a comment like the one Paolo added when he
made the same change in kvm_read_guest_virt?
See commit 353c0956a618 ("KVM: x86: work around leak of uninitialized
stack contents (CVE-2019-7222)").